Oscillator



July 7, 19x42- A. c. s'rocKER 2,288,856

OSCILLTOR Filed July 51, 1940 SECT/OMS 7'00 CLOSE FOTHT//V /N DEGREES ttorneg Patented July 7, 1942 UNITED STATES PATENT OFFICE OSCILLATOR Arthur C. Stocker, Haddon Heights, N. J., assignor to Radio Corporation o1' America, a corpora.-

tion of Delaware Application July 31, 1940, Serial No. 348,918 s claims. (Cl. 1v1-242) This invention relates to straight line frequency oscillators, and more particularly to a variable inductor for use in an oscillatory circuit for producing oscillations of variable frequency having a constant or straight-line frequency versus angular-rotation characteristic.

In an application Serial No. 279,544, filed June 16, 1939, now Patent No. 2,248,472, July 8, 1941 and entitled Frequency generator, I have described a frequency generator employing a Vernier or interpolation oscillator the function of which is to generate oscillations which are variable over a small range of frequencies. In its original form, this oscillator was tuned by a variable capacitor which, as is well known, may be given any desired frequency versus rotation characteristic either by cutting the plates or by bending sections of one or more plates, or both. It will be appreciated that it is a difficult task to adjust a Variable condenser in this manner to a straight line frequency characteristic with any degree of accuracy. However, in order to assure accurate reading of the dial without the necessity of an individual dial scale calibration, it is highly desirable to provide an oscillator for this purpose in which the incremental change of frequency for each division of the indicating dial is a constant, and in which the frequency characteristic of the variable element is readily adjustable to compensate for various circuit conditions. The principal object of this invention is to provide such a circuit employing a variable inductor in which the frequency versus angular-rotation characteristic is linear within an error of less than .005 percent. Other objects of this invention include the provision of means for an improved variable inductor; the provision of means for adjusting the frequency characteristic of a Variable inductor to provide the desired linearity; and to provide means for adjusting the axis of rotation of one section of a coil with respect to another section of the same coil.

In brief, the above objects are accomplished in accordance with this invention by means of a pair of serially-connected, mutually-coupled double D-type coils which are mounted coaxially, one of which is rotated about an axis parallel to but spaced from the common axis of the coils.

This invention will be better understood from the following description when considered in connection with the accompanying drawing in which Figures l and 2 are Views illustrating a double D-type coil; Figure 3 is a plan view of two double D-type coils mounted in their operatlustrating the displaced axis of rotation on one of the coils; Figure 5 is a cross-sectional view of such a coil illustrating means for adjusting a portion of the coil; Figure 6 is a. view illustrating an arrangement for adjusting the axis of rotation of one of the coils; Figures 7 and 8 are curves illustrating the deviation from the desired linear characteristic produced by certain improper adjustments of the coil assembly; and Figure 9 is a circuit diagram illustrating a circuit suitable for use in connection with this invention. Similar reference characters will be ing positions; Z'igure 4 is a schematic drawing il- 55 applied to similar elements of the several figures.

The term double D-type coil as herein used is intended to refer to a coil of the type illustrated in Figs. 1 and 2, to which reference is now made. These coils are wound on a cylindrical form Il having oppositely disposed longitudinal slots in one end. The wire I3 is Wound in figure-ofeight fashion half way round the circumference of the form in a given direction, then through the opposed slots across the diameter of the form and around the other half of the form in the opposite direction. As used in this specification, the term diametrical section" refers to that portion of the winding which lies substantially parallel to the coil diameter. The term circumferential section refers to that portion of each coil which is wound around the outer circumference of the coil form. A complete variable inductor comprises two such D-type coils connected in series, and mounted coaxially in mutual coupling position, as illustrated in Fig. 3. This produces a two-terminal variable inductor of the type commonly known as a variometen but it will be appreciated that the conventional variometer is not a straight line frequency device. The present arrangement distinguishes over the usual variometer arrangement in that the rotor coil is mounted for rotation about an axis X-X which is parallel to but displaced from the common axis of the coils Y-Y. The direction of displacement preferably lies along the winding diameter of the coil. The amount of displacement is determined by trial, as will be explained hereinafter.

I have found that the most nearly perfect range of operation lies between the angular limits of rotation which are approximately 45 and from the position in which the winding diameters of the two coils are parallel. This is graphically illustrated in Figure 4 in which the stator coil is represented by the circle i5, while the rotor coil in its approximately extreme positions is represented by circles I'l and Il'. It

will be noted that, when the two coils are in a position of maximum coupling, their winding diameters will be parallel and they will then be coaxially positioned. After rotation about the displaced X-axis, however, the rotor is no longer coaxially disposed with respect to the stator coil.

In order to achieve the required high degree of linearity, I have found it helpful to provide means for independently adjusting the spacing between the diametrical sections of the coils with respect to the spacing betwen the circumferential sections of the coils. One method of accomplishing this adjustment is illustrated in Fig. 5 to which reference is now made.

The coil form is shown partially in crosssection in Fig. 5 to illustrate the diametrical section of the coil. A small block of insulating material' I9 is mounted within the coil form and fastened to or hooked over the outer edge of the diametrical section of the coil. A set screw 2| provided with a lock nut 23 is mounted in an i end plate 25, on which the coil form is mounted.

The'set screw engages the insulating block I9 and makes possible its axial adjustment. It is only necessary to provide a very small adjustment of this portion of the coil, and I have found it suftlcient to obtain this adjustment by merely increasing the tension on the wires. It will be observed that this adjustment does not affectthe position of the coil turns on the outer cir. cumference of the coil form.

As indicated above, the two serially-connected double D coils must be carefullyadjusted to provide an accurate straight line frequency characteristic. Figs. 7 and 8 illustrate the nature of the deviation which is produced by the adjustment of the. cross wires which constitute the diametrical section of the coils and the degree of eccentricity, that is, the displacement of the axis of rotation. The solid curved line of Fig. 7 illustrates the frequency versus rotation characteristic when the cross wires are too closely spaced. If the measured characteristic is of this type, the set screw 2| should be tightened up to decrease the coupling between the diametrical sections of the coils. The curved dotted line illustrates the nature of the frequency characteristic obtained when the cross wires are spaced too far apart.

At the same time it is necessary tov consider the degree of eccentricity of the axis of rotation. The curved solid line of Fig. 8 illustrates the nature of the frequency characteristic of the coil when the axis of rotation is displaced to far from tic may be obtained by a careful adjustment .of

the displacement ofthe center of rotation` and the coupling.

It will be appreciated that the range of inductance, and hence the range of frequency throughout which the linear characteristic is obtained, .is determined by the number of turns on each of the coils and the spacing between 'coils themselves. Adjusting the axial distance between the two coils as a whole does not appreciably affect the frequency characteristic of rotation, and hence such an adjustment may be made to increase or decreaseA the range of frequency to the desired value. Large changes in the vfrequency range, however, should be made by a suitable change in the number of turns of the two coil sections.

One method for obtaining an accurate control 1 the fixed stator coil.

of the displacement of the axis of rotation is illustrated in Fig. 6.'

The coil form Il, containing one of the two double D coils I3, is mounted by a suitable bracket 21 on a slidable member 29 which is in turn supported by a supporting member 3| which is mounted on the rotatable shaft 33. 'I'he slidable member 29 is held in a xed position by means of a pair of adjusting screws 35, 3l which may be used to move the slidable member in a. direction perpendiculanto the axis of the shaft 33. This causes the coil form and coil to rotate about an axis which is displaced from and parallel to the axis of the shaft, as required.

The high degree of accuracy whichmay be obtained with a coil of this type is illustrated by the fact that in early development work on the system the limit of accuracy rst reached was found to be the non-linearity of the worm drive of the dial mechanism itself, even though the best dial then obtainable was used. After improving the accuracy of the dial mechanism itself so that the angular rotation of the shaft was made to conform more perfectly to the indicated angular rotation', the measured characteristic was improved to such a degree that the greatest deviation was of the order of 2 or 3 cycles per second in an oscillator whose frequency was approximately 170 kilocycles per second.

While there are many circuits suitable for use in connection with a variable inductor of the type described above, an electron-coupled oscillator is illustrated in Fig. 9 which has proved to be satisfactory. In order to avoid bringing out a tap on one of the D coils, or to reduce the overall change of frequency, if desired, the variable inductor comprising the two serially-connected D coils 39 and 4| may be connected in series with a xed inductor 43 having a suitable tap 45. One end of the D coils is connected to ground and the free end of the series inductor 63 is coupled to the control grid of an oscillator tube 41. The tap 45 of the series coil I3 is connected to the cathode of the oscillator tube 41. The entire inductor assembly may be tuned by means of a shuntcapacitor 49, if desired. The suppressor grid of the oscillator is grounded. Output is taken from the plate electrode.

It will be appreciated that, in certain instances where a maximum range of frequency is desired, the series coil 43 will not be needed, in which case the tap may be tak'en from a suitable point on It will be recognized, how- 1 ever, that the coil will have to be readjusted each time the series coil is changed. Any convenient means for making connections to the rotor coil may be utilized. Slidable contacts or pigtail connections may be used, although, where extreme accuracy is desired, I have found that the former is to be preferred.

I have thus described an oscillator having a linear'frequency characteristic to a high degree of accuracy, which includes a unique variable inductor and-which is provided with means for adjusting the change of frequency as a function of angular rotation to produce the desired characteristic. i l

I claim as my invention:

1. A variable inductor comprising a pair of serially-coupled double D coils, means for mounting said coils in spaced mutually coupling relation, and means for rotating one of said coils about an axis displaced from the axis of said coils and parallel thereto, the displacement of said axisof rotation being adjusted to a. value which produces a linear-frequency versus rotation characteristic for said coils.

3. A variable inductor having a straight line frequency characteristic comprising a pair of double D coils positioned in a normally coaxial position and connected in series, means for rotating one of said coils about an axis parallel to said common axis but displaced therefrom along the winding diameter of said coil, the amplitude of said displacement being adjustable to control the frequency characteristic of said coll as a function of the angular displacement between said coils.

4. A variable inductor comprising a pair of coils, each coil comprising a cylindrical supporting form having a slotted portion, and a winding consisting of turns wound around said form and through said slot in figure-eight fashion, means mounting said coils in coupling relation, and means for independently adjusting the coupling between portions of said coils lying on the surface of said forms and the diametrical portions of said coils lying in said slots.

5. The device described in claim 4 in which said mounting means includes means for mounting said coils normally along acommon axis and for rotating one of said coils about an axis displaced from said common axis.

ARTHUR C. STOCKER. 

